WATER-ADSORPTION CAPACITY OF THE SOLID ADSORBENTS TENAX-TA, TENAX-GR, CARBOTRAP, CARBOTRAP-C, CARBOSIEVE-SIII, AND CARBOXEN-569 AND WATER MANAGEMENT-TECHNIQUES FOR THE ATMOSPHERIC SAMPLING OF VOLATILE ORGANIC TRACE GASES

The solid adsorbents Tenax TA, Tenax GR, Carbotrap C, Carbotrap, Carbosieve Sill, and Carboxen 569, all of which are commonly used for sampling of volatile organic compounds (VOCs) from the atmosphere, were tested for their capacity to trap water vapor by a new experimental approach, Instead of performing breakthrough measurements, water uptake was determined gravimetrically until saturation, Test atmospheres at 10, 20, 30, and 40 degrees C and 20, 40, 60, 80, and 100% relative humidity (RH) in a temperature (T)- and humidity-controlled climate chamber were collected onto sampling cartridges containing beds of the above-listed adsorbents, This technique allowed us to obtain information on the dependency of the water uptake on sampling volume, T, and RH and to deduce strategies for water management. Tenax TA, Tenax GR, Carbotrap, and Carbotrap C showed low water trapping of generally less than similar to 2-3 mg of water/g of adsorbent under the highest humidity conditions tested, In contrast, the carbon molecular sieve type adsorbents Carboxen 569 and Carbosieve SIII exhibited substantially higher water trapping capacity, with up to similar to 400 mg of water/g of adsorbent under the highest RH conditions tested, For Carboxen 569 and Carbosieve SIII, water saturation occurs relatively slowly; the water saturation point is reached only after the sampling of volumes that are much higher than typical volumes collected in ambient VOC sampling, For both of these adsorbents, it was shown that the amount of water adsorbed at a given temperature depends mainly on the RH levels, For a multibed adsorbent cartridge, the overall water adsorption could be calculated reasonably from the expected contributions of the individual components, Possible measures for minimizing the amount of water trapped are, for example, collecting small sampling volumes, minimizing the amount of adsorbent in the sampling tube, moderate heating of the adsorbent tube during sampling, or including a dry purge step before thermal desorption.